LED fireworks on an Arduino Uno

// LED fireworks: rockets climb a strip of LEDs, burst into sparks that fly
// both ways and fade, and a piezo whistles on the way up and pops at the top.
// Press the button for the grand finale.
//
//   strip     30 WS2812B LEDs, DIN <- 330 ohms <- pin 6 (stand it up straight)
//   piezo     pin 8 -> piezo -> GND
//   button    pin 2 -> button -> GND        (INPUT_PULLUP): the finale
//
// Hang the strip up a window or along a fence post, LED 0 at the bottom. A
// rocket is one bright dot with a short tail that slows as it climbs, the way
// a real one does, and bursts at a random height. The burst throws sparks of
// one color up and down from that point, each slowing and fading, with a few
// white glitters at the end.
//
// Everything moves on a frame clock: each frame moves the rocket, ages the
// sparks and draws them all, then returns, so the button is read between
// frames and is never missed.
//
// Real fireworks are for the professionals. These ones are safe for any age,
// any weather and any noise rule.
//
// Things to try. Press the button for the finale. Add colors to COLORS. Make
// the bursts bigger with SPARKS.

#include <Adafruit_NeoPixel.h>

const int LEDS = 30;
const int PIEZO_PIN = 8;
const int BUTTON_PIN = 2;
const int SPARKS = 8;

Adafruit_NeoPixel strip(LEDS, 6, NEO_GRB + NEO_KHZ800);

const uint32_t COLORS[] = {0xFF2020, 0xFFFFFF, 0x2050FF, 0xFFC020, 0x30FF60, 0xFF40FF};
const int SHADES = sizeof(COLORS) / sizeof(COLORS[0]);

// Positions and speeds in 1/16ths of an LED, so things can move slowly.
int rocket = -1, rocketSpeed = 0, burstAt = 0;
int sparkPos[SPARKS], sparkSpeed[SPARKS];
uint8_t sparkLife = 0;
uint32_t sparkColor = 0;
unsigned long nextLaunch = 1000;
int launches = 0;
int finale = 0;   // rockets left in the finale

void launch() {
  rocket = 0;
  rocketSpeed = random(10, 14);
  burstAt = random(LEDS / 2, LEDS - 3) * 16;
  launches++;
  Serial.print("Launch ");
  Serial.println(launches);
}

void burst() {
  sparkColor = COLORS[random(0, SHADES)];
  for (int i = 0; i < SPARKS; i++) {
    sparkPos[i] = rocket;
    sparkSpeed[i] = (i % 2 ? 1 : -1) * random(3, 12);
  }
  sparkLife = 255;
  rocket = -1;
  tone(PIEZO_PIN, 140, 60);   // pop
}

void setup() {
  Serial.begin(9600);
  pinMode(BUTTON_PIN, INPUT_PULLUP);
  strip.begin();
  strip.setBrightness(200);
  strip.show();
  randomSeed(analogRead(A0));
  Serial.println("Fireworks ready: press the button for the finale");
}

void loop() {
  unsigned long now = millis();
  if (digitalRead(BUTTON_PIN) == LOW && finale == 0) {
    finale = 6;
    Serial.println("Grand finale!");
  }
  if (rocket < 0 && sparkLife < 120 && (now >= nextLaunch || finale > 0)) {
    launch();
    if (finale > 0) finale--;
    nextLaunch = now + random(1500, 3500);
  }

  strip.clear();
  if (rocket >= 0) {
    rocket += rocketSpeed;
    if (rocketSpeed > 4 && random(0, 3) == 0) rocketSpeed--;   // slowing
    tone(PIEZO_PIN, 1500 + rocket * 2, 20);                     // whistle
    int at = rocket / 16;
    strip.setPixelColor(at, 255, 200, 120);
    if (at > 0) strip.setPixelColor(at - 1, 60, 30, 0);
    if (rocket >= burstAt) burst();
  }
  if (sparkLife > 0) {
    uint8_t r = (uint8_t)(sparkColor >> 16) * sparkLife / 255;
    uint8_t g = (uint8_t)(sparkColor >> 8) * sparkLife / 255;
    uint8_t b = (uint8_t)sparkColor * sparkLife / 255;
    for (int i = 0; i < SPARKS; i++) {
      sparkPos[i] += sparkSpeed[i];
      if (sparkSpeed[i] > 1) sparkSpeed[i]--;
      if (sparkSpeed[i] < -1) sparkSpeed[i]++;
      int at = sparkPos[i] / 16;
      if (at >= 0 && at < LEDS) {
        bool glitter = sparkLife < 60 && random(0, 4) == 0;
        strip.setPixelColor(at, glitter ? strip.Color(255, 255, 255) : strip.Color(r, g, b));
      }
    }
    sparkLife = sparkLife > 8 ? sparkLife - 8 : 0;
  }
  strip.show();
  delay(25);
}